The present invention is directed to a pressing tool for forming a blank of compressed sinter material, and to a cutting bit formed by sintering the blank.
Cutting bits such as reversible cutting plates can be produced by sintering from pressed blanks. Such blanks shrink during sintering at a so-called shrinkage ratio which is a function, in particular, of the material being sintered.
A pressing tool for pressing the blanks includes a mold member cooperating with an upper die and a lower die. Generally the lower die, and a recess in the mold member provided to guide it, have large cross sections which correspond to the bottom surface of the cutting bit after allowances are made for the shrinkage ratio. For the upper die, which is also guided in the mold member, the cross section of the upper die and the recess provided to guide it correspond (again after allowing for the shrinkage ratio) to a surface defined by the upper corner points of the cutting bit and to imaginary straight lines connecting them.
Cutting bits may have flanks that are inclined with respect to the bottom side. To make the blanks for such cutting bits using a pressing tool which employs an upper die and a lower die guided in a mold member, the mold member has transition faces, from the upper guide recess to the lower guide recess, which correspond (disregarding the shrinkage ratio) to the flanks of the cutting bits and which are inclined at the same clearance angle. Assuming that the flanks are planar, in the mold member the transitions from the perpendicular sides of the upper guide recess to the corresponding transition faces are formed by straight edges or intersections at angles to one another, respectively.
A so-called negative cutting bit lacks a clearance angle, i.e. has flanks which are perpendicular to the bottom side, with or without inclined cutting edges. A so-called positive cutting bit has a clearance angle .alpha.&gt;0 and cutting edges along an imaginary straight line, i.e. not inclined, the cutting edges being continuous from corner to corner. Blanks for such negative and positive cutting bits can easily be pressed into the final or finished form, disregarding the shrinkage ratio. However blanks pressed with known pressing tools for positive cutting bits which have a clearance angle .alpha.&gt;0 and which have cutting edges inclined at an angle .lambda., with such cutting edges having a non-linear shape between two adjacent corners when seen from the top, have a section of roughly 1 mm in height at their upper side which has peripheral or flank faces without a clearance angle as a result of the perpendicular peripheral or guide surfaces of the upper die. The flanks are overdimensioned and follow at the lower edge of these peripheral faces. To finish the sintered cutting bits, the flanks must still be ground by essentially an amount of 1 mm .times. sin .alpha..
Attempts have already been made to provide the upper die with a slope which is inclined at a clearance angle .alpha. between the highest and lowest points of the lines defining the pressure face of the upper die. However, such attempts have resulted in a local increase in pressure at the end of the pressing stroke, in conjunction with the transition faces of the mold member, which faces slope at the same clearance angle, due to clamped-in sinter material. A danger of destruction exists, particularly for the upper die.